The present invention relates to an improved self-inductance sensor used for measuring the change in circumference of an object.
The field of plethysmography, i.e., the study of the change in size of an organ or limb, often employs inductive sensors as the measuring device. A variety of such sensors have been disclosed in various patents to Goldberg et al., including U.S. Pat. No. 3,560,845. The use of such a sensor to measure cross-sectional area changes in the torso, and thus respiration volume, has been discussed in Milledge, J.S., and Stott, F.D., "Inductive Plethysmography -- A new Respiratory Transducer", Proceedings of the Physiological Society, January 1977, pages 4-5. By measuring the simultaneous changes in airflow, the changes in the signals from various types of chest and abdominal respiration sensors can be weighted and summed in order to provide an independent measurement of respiration volume. Such calibration procedures were described by Shapiro, A. and Cohen, H.D. (1965) "Transactions of the New York Academy of Science", Vol. 27, page 634. Such techniques were further explored in Konno, K. and Mead, J.(1967) "Journal of Applied Physiology", Vol 22, page 407. In U.S. Pat. Nos. 4,308,872 and 4,373,534 Watson et al. further describe the use of inductive plethysmography sensors which measure cross-sectional area.
The use of an inductive sensor which circumscribes the torso has been found to have certain inherent disadvantages. In theory, an inductive sensor which fully encircles the torso is advantageous in respiration plethysmography because it measures cross-sectional area, not circumference, and it is that area which is directly proportional to respiration volume. However, sensors containing conductors which fully circumscribe the entire torso are subject to error from calibration inaccuracies, displacement of the sensor, deformation of the sensor for reasons other than respiration, and changes in output due to different body positions. These errors tend to be of greater magnitude than any errors due to measuring cross-sectional area rather than circumference. Also, to complete the current loop, electrical connections must be made to the conductor at each end each time that the sensor is placed on the patient. This is a difficult and cumbersome procedure which is subject to frequent failure, and requires the use of a sensor fitted to the size of the patient.